Magnetic sensors are increasingly important in various industries. In the automotive industry sensors in particular, various sensors such as parking sensors, angular sensors, ABS (Automatic Braking System) sensors and tyre pressure sensors can be found in modern vehicles for improving comfort and safety. Magnetic sensors are particularly important in automotive applications, because magnetic fields penetrate easily through most materials. Magnetic sensors are also highly insensitive to dirt, unlike for example optical sensors.
Several different magnetic sensor technologies are currently available, such as sensors based on the Hall effect or the magnetoresistive effect. Anisotropic magnetoresistive (AMR) and giant magnetoresistive (GMR) sensors are particular examples of sensor types based on the magnetoresistive effect. Hall effect sensors can be integrated monolithically into integrated circuits, which makes them cheap, but they are also known for their low sensitivity and consequent inaccuracy. AMR sensors, while having a much higher sensitivity compared to Hall effect sensors, require more fabrication steps because they cannot be integrated monolithically, making a total sensor system more expensive. AMR sensors can be deposited, usually by sputtering of Ni80Fe20 on a separate die or on top of a monolithic structure. An annealing process, sometimes in a magnetic field, is often used for increased stabilisation of the magnetic state in the magneto-resistive material.
GMR sensors typically have a higher sensitivity than AMR sensors. However, a GMR sensor consists of various thin layers and critical interfaces. The technology required to fabricate such sensors is considerably more complicated and expensive. Furthermore, due to the thin multiple layers making up a GMR sensor, the operating temperature range is also limited. Therefore often AMR sensors are chosen as a good compromise in magnetic sensor applications.
An AMR sensor (101) is sketched in FIG. 1 left. The AMR sensor is supplied by a sense current Isense that can be extracted from for example a reference voltage Vref in series with a resistor R. A typical AMR transfer function, defined as the AMR sensor resistance, RAMR, as a function of the applied (or external) magnetic field, Hext, is displayed in FIG. 1 right (102). The transfer function is symmetrical with respect to the y-axis and consequently has vanishing sensitivity near the zero crossings of Hext. This strongly hampers an accurate detection of zero-field crossings: for such a symmetrical transfer curve, electronic noise and other disturbing electronic signals have a large impact at and around Hext=0.
One known way to tackle this problem is the addition of a coil on top of the AMR sensor, see FIG. 2 left (201). When a DC current (Ibias) is driven through the coil, an additional field Hbias is generated in the AMR sensor. The bias point of the AMR sensor is now shifted from 0 (202) to Hbias (203) on the AMR transfer function, see FIG. 2 right. The AMR is now sensitive at zero Hext and its response to a sinusoidal Hext of the AMR sensor might look like the one depicted in FIG. 2 right.
Unfortunately, a difference in positive and negative half periods is visible due to the not purely anti-symmetric transfer curve that is generally obtained by the DC bias. Therefore, the zero-crossings of the external field do not coincide with the average value of the resistance of the sensor. So, this average value can not be used to detect the zero-field crossings. Moreover, the deviation of the actual zero-field crossings from this average AMR resistance level depends from the amplitude of the external field (and also from the size of the bias field, the shape of the transfer curve etc.) This hampers a robust and reliable detection of the zero field crossings from a single AC AMR output signal.
Because the transfer curve is not purely anti-symmetric (about the point 203), even order distortion components arise. In other words, if a sinusoidal field Hext is applied to the sensor, a spectral analysis (FFT) on the AMR sensor output resistance shows not only the odd harmonics from a purely anti-symmetric transfer curve but also some even harmonics resulting from the (shifted) symmetrical nature of the transfer curve.
One known approach to address these problems is the use of barber poles in the AMR sensors. These force current flow in the sensors to follow a certain direction. A disadvantage of barber pole type sensor elements is that the output curve changes its sign when the magnetization flips everywhere in the element, for example due to a large field spike. This would lead to a wrong interpretation of the sign of the applied field. A reduced sensitivity results if only a small part of an element flips. When this effect is not equal in the two elements, which is most likely, it leads to a NOT purely anti-symmetric output and this leads to ‘distortion’ of the zero crossings. The above described wrong states might be permanent until a next large field pulse occurs.
There is therefore a need for an AMR sensor configuration in which zero point crossings can be detected more easily and reliably.